Dissecting the Nautilus

[Buckle-meister]

No, I'm not getting them...yet.

I am interested in learning about B&W's flagship Nautilus speakers and so I downloaded the manual for them. However, not being particularly knowledgeable about speakers, there were a number of bits I didn't understand (or thought might be the truth dressed up to make it sound more impressive). With this in mind, I'd be very appreciative if anyone could enlighten me on any of the following quotes from the manual:

Laurence [Dickie] had been experimenting with drivers mounted in the curved surface of a cylinder and encountered results not dissimilar to those of the dipole. Namely, that external cabinet effects could be virtually eliminated and the intrinsic sound of the unit heard.

He used a ring magnet outside the coil with a thin-walled cylindrical pole piece to allow a smooth transition from dome to enclosure. Only one type of enclosure will provide absolute freedom from aberration – the infinite pipe or waveguide. Excitingly, it became possible to imagine that an entirely waveguide-based system could actually work. Research showed that the exponentially tapered pipe was an even better absorber than the cylinder. So complete was its absorbing action that the pipe could be left open or closed.

What's an infinite pipe/waveguide? Is it just a fancy name for a tapered tube as opposed to a parallel walled tube discussed in the first paragraph of the quote?

It was decided that the system should be four-way with 300mm (12in), 100mm (4in), 50mm (2in) and 25mm (1in) units – all mounted in tapered lines within a diffraction limiting enclosure.

Would I be right in saying that a diffraction limiting enclosure simply means that the front face of the speaker enclosure is curved rather than flat?

The drive for sonic purity is reinforced by using an active crossover design allowing separate amplification of each drive unit, cutting out component crosstalk and driver inter-reaction.

What's component crosstalk and driver inter-reaction and perhaps more importantly, are their effects audible?

Exponential line loading is used for all four drive units, primarily to achieve freedom from resonance and reflection, but also for the damping effect on the fundamental resonance in the low frequencies.

What's exponential line loading?

All voice coils are wound on polyimide formers to eliminate eddy-current losses, which are particularly serious at high frequencies.

What's eddy-current? Is it some form of 'stray' current? In addition, why is it serious at high frequencies (I'm guessing that it'll have something to do with burning up the fragile tweeter)?

A 9.5kg (21lbs) magnet with a 100mm (4in) voice coil acts as the massive motor of the 300mm (12in) bass unit. This, when used in the exponential line enclosure, results in a high-pass behaviour so over-damped that the traditional second-order characteristic is replaced by two distinct first-order slopes and no stored energy.

Can anybody explain, in layman's terms, what's happening here?

A rare earth magnet assembly with hollow pole is used to minimise the obstruction to the rear radiation from the diaphragm.

A rare earth magnet?

All drive units are completely mounted on silicone rubber O-rings to decouple them from the cabinet.

I've read in the past that, for example, speaker spikes do not decouple a speaker from the surface it stands upon and that actually, this is impossible due to Newtons 3rd law of motion which states that for every action there is an equal and opposite reaction. How then can the drivers in the Nautilus' be decoupled from the cabinet?

The division of the signal into the four required bandwidths is accomplished in the Nautilus Active Crossover via totally nonresonant circuitry

Nonresonant circuitry? Is the effect of 'resonant circuitry', which presumably is in the vast majority of speakers, audible?

In general it will be found that Nautilus gives optimum results when “toed-in” to a greater extent than in previous systems, set typically at an angle of between 60° and 90°. This is due to the smooth, wide dispersion of Nautilus which is capable of increasing the relative significance of the side-wall reflection.

Why does B&W suggest that the Nautilus' be toed-in more than 'usual', i.e. more akin to a monitor-type speaker, given that it exhibits a smooth, wide dispersion (frequency response of the speaker lies within 2dB of the response on reference axis over 60° horizontally and 0° vertically)? Aren't monitor-type speakers usually toed in because they don't have a smooth wide dispersion?

Each Nautilus will require separate amplifier channel for each drive unit. The gain and phase of each must be identical.

If I understand rightly, the gain for each power amplifier needs to be identical so that no individual driver is artificially being made louder or quieter than another. Correct?

In addition, would powering a pair of Nautilus' be achieved simply by connecting all eight (four drivers per speaker) power amplifiers to a single pre-amplifier, i.e. just like a regular separates system albeit wth more power amplifiers, and if so, would this automatically ensure that the gain and phase of individual power amplifiers was identical?

 

[WmAx]

What's an infinite pipe/waveguide? Is it just a fancy name for a tapered tube as opposed to a parallel walled tube discussed in the first paragraph of the quote?

Sounds to me as if they are trying to cross reference the physical behaviour of one event to a another, resulting in nifty name to use in marketing. For example, here, the combination of internal damping material and extended tapered tube acts identically to an open rear(infinite baffle), so far as reflections/resonances are concerned. But this is quite the bit of marketing hooplah, also, as you don't need tapered chambers if you use the correct damping material(s) with sufficient acoustical co-efficient at the relevant frequencies.

Would I be right in saying that a diffraction limiting enclosure simply means that the front face of the speaker enclosure is curved rather than flat?

Yes. Also, it has minimum surface area. The actual effectiveness of such depends on the specific measurements of the surfaces/curves in relation to the wavelengths in air.

What's component crosstalk and driver inter-reaction and perhaps more importantly, are their effects audible?

They are referring to the tendency for components in a passive crossover, mainly the inductors, to radiate electromagnetic energy and pick it up from the other inductors in the circuit. Audible? Perhaps with the right test signal, and with a crossover layout that was specifically intended to to do this.

What's exponential line loading?

Probably the same answer as to your first question. But I am not sure what they mean, either, as in the end, it's just a sealed enclosure, but with a dense 100 percent fill of damping material in the folded length. It probably acts as a somewhat larger sealed enclosure volume than in reality. Probably similar to the practice of using damping material in a sealed enclosure to increase effective volume, but taken to a greater degree of effect. To know exactly what to expect in terms of apparent volume increase, you would need to (1) measure the system vs. raw volume in a normal enclosure [or] (2) ask an expert on line loading such as Martin King over at http://www.quarter-wave.com .

What's eddy-current? Is it some form of 'stray' current? In addition, why is it serious at high frequencies (I'm guessing that it'll have something to do with burning up the fragile tweeter)?

This is jnuetron's territory. This is the first time I ever heard this brought up in relation to speaker crossovers.

I've read in the past that, for example, speaker spikes do not decouple a speaker from the surface it stands upon and that actually, this is impossible due to Newtons 3rd law of motion which states that for every action there is an equal and opposite reaction. How then can the drivers in the Nautilus' be decoupled from the cabinet?

It's not impossible to de-couple a speaker from a surface. But it's impossible for a solid metal spike to do it. To de-couple, the material layer(s) used will have a resonant frequency below the intended frequency to be filtered. This will act as a 2nd order high-pass filter. Now, the consequence is that the energy that would have been distributed into the secondary mass is now retained in the original source, thus causing a higher degree of vibration in that device, and/or, the suspension will disperse some of this as thermal energy, the amount related directly to the level of dampening.

Nonresonant circuitry? Is the effect of 'resonant circuitry', which presumably is in the vast majority of speakers, audible?

Lost me. I don't what is supposed to resonant, unless they are talking about the resonant properties(due to filter slope Q) of the actual crossover slopes. Sounds like a long shot from the marketing department, to me.

Why does B&W suggest that the Nautilus' be toed-in more than 'usual', i.e. more akin to a monitor-type speaker, given that it exhibits a smooth, wide dispersion (frequency response of the speaker lies within 2dB of the response on reference axis over 60° horizontally and 0° vertically)? Aren't monitor-type speakers usually toed in because they don't have a smooth wide dispersion?

I don't know. Is there a frequency response vs. axis plot available that I can view? But if it does in fact have such behaviour, then it's a subjective sound preference of how much you toe them in, not a rule. As for [typical] studio monitors, yes, you are correct.

If I understand rightly, the gain for each power amplifier needs to be identical so that no individual driver is artificially being made louder or quieter than another. Correct?

Correct. But you don't need the same model amplifiers. You need only a volume control/trim potentiometer on each amp and a multi-meter, along with some test signals on a CD to match the effective gain.

In addition, would powering a pair of Nautilus' be achieved simply by connecting all eight (four drivers per speaker) power amplifiers to a single pre-amplifier, i.e. just like a regular separates system albeit wth more power amplifiers, and if so, would this automatically ensure that the gain and phase of individual power amplifiers was identical?

Yes. My understanding is that the Nautilus comes with an electronic crossover box with stereo inputs. Feed it from a pre-amp, or your IPOD. Whichever suits you.

-Chris

 

[Buckle-meister]

The actual effectiveness of [a curved surface] ...depends on the specific measurements of the surfaces/curves in relation to the wavelengths in air.

So, would it then follow that to maximise its effectiveness, the curvature of the cabinet around each driver would be different, given that each driver reproduces a separate range of frequencies (not saying that B&W have done this)?

Audible? Perhaps with the right test signal, and with a crossover layout that was specifically intended to to do this.

I'll take that as a 'probably not under what would be considered normal operating conditions'.

To de-couple, the material layer(s) used will have a resonant frequency below the intended frequency to be filtered. This will act as a 2nd order high-pass filter. Now, the consequence is that the energy that would have been distributed into the secondary mass is now retained in the original source, thus causing a higher degree of vibration in that device...

So, does it then follow that in any speaker in which the drivers are decoupled from the cabinet, the drivers themselves must be made of a more rigid material than might typically be used in order to accomodate vibrations that ordinarily would be dispersed into the surrounding enclosure? If so, could this at least partially explain B&W's use of aluminium for the drivers over, say, paper?

Is there a frequency response vs. axis plot available that I can view?

Not that I know of. Sorry.

You need only a volume control/trim potentiometer on each amp and a multi-meter, along with some test signals on a CD to match the effective gain.

To what accuracy would you set the gain in order to consider all amps as outputting equally?

My understanding is that the Nautilus comes with an electronic crossover box with stereo inputs. Feed it from a pre-amp, or your IPOD. Whichever suits you.

Hmm. So are you saying that you wouldn't connect all the power amps directly to the pre amp as in normal separates, but that all the components connect separately into the Nautilus' crossover and it then sorts everything out from there (I must admit to wondering how one would connect eight power amps into a pre amp from the point of view of never having seen so many pre-outs on a pre amp before)

http://www.quarter-wave.com.

Man, those are some strange looking speakers in the gallery from the above website. Some really nice looking ones too though.

Whilst looking through the gallery, I kept on seeing various speakers described as having full-range drivers yet typically there appeared to be only one driver which itself looked to be no more than 6" to 8" in diameter. How can the speakers be considered full-range?

Thanks mate.

 

[Haoleb]

A rare earth magnet?

Rare earth magnets have a much stronger magnetic field for given size than regular magnets. I believe most consumer level speakers use ceramic magnets.

http://en.wikipedia.org/wiki/Rare-earth_magnet

 

[AVRat]

As far as amps, you’d want different powered amps that have a voltage gain which is usually around 29dB and volume control as WmAx mentioned. Amps rated somewhere around 35W, 70W, 140W, and 280W for a total of 525W should work.

 

[Buckle-meister]

Ok, I'm a bit confused by the whole 'active crossover' setup. I know how to wire a typical setup and think I understand the way it works with a speakers passive crossover. With an active crossover such as the Nautilus', how is everything wired together, i.e. in what order?

Amps rated somewhere around 35W, 70W, 140W, and 280W for a total of 525W should work.

They may well be enough, but B&W recommend amps that deliver from 100W to 500W.

 

[jaxvon]

Ok, I'm a bit confused by the whole 'active crossover' setup. I know how to wire a typical setup and think I understand the way it works with a speakers passive crossover. With an active crossover such as the Nautilus', how is everything wired together, i.e. in what order?



They may well be enough, but B&W recommend amps that deliver from 100W to 500W.

With an active crossover, you feed the crossover the signal directly from your pre/pro. From there, the crossover splits the signal into multiple signals (four in the case of the Nautilus), each of which goes to an amplifier and finally to a driver. This way you get one amplifier per driver.

 

[furrycute]

All this technical mumbo jumbo is giving me a real headache...

Has anyone here on audioholics ever been lucky enough to audition a pair of these Nautilus speakers? Or better yet, anyone rich enough to own a pair?

If anyone does, please give us your impressions. Mighty appreciated.

 

[Buckle-meister]

With an active crossover, you feed the crossover the signal directly from your pre/pro. From there, the crossover splits the signal into multiple signals (four in the case of the Nautilus), each of which goes to an amplifier and finally to a driver. This way you get one amplifier per driver.

Thanks Jaxvon. Does this mean that an active crossover is an offboard device, i.e. physically separated from the speakers, and that one could, for example, stick the two from the Nautilus in a hifi rack?

All this technical mumbo jumbo is giving me a real headache...

Hey! I like learning about all this 'mumbo-jumbo'.

Has anyone here on audioholics ever been lucky enough to audition a pair of these Nautilus speakers?

I'm not sure that it is possible to audition the Nautilus' as they're built to order.

 

[furrycute]

The crossover has built in frequency cutoff points. For example, above a certain frequency, the signal is sent to the tweeter section. Below a certain frequency, that signal is sent to the woofer section. Of if your speaker is three way, below a certain low frequency cutoff point, that signal is sent to the bass woofer section.

Most speakers have built in passive crossovers (passive meaning the process of sorting out which frequencies go to which driver is done passively, it doesn't need to be plugged into an outlet).

Many high end manufacturers sell active crossovers (meaning it's a separate component that needs to be plugged into an outlet). But before you can use these active crossovers with your speakers, you need to physically disbale or remove your speakers' built in passive crossovers (if they have them, most speakers do).

I don't understand a lot of the physics behind it all. But from what I've read, active crossovers do have better sonic characteristics than passive crossovers. It has to do with those slopes, high order cutoffs, etc., which I am absolutely cluless about.

And yes, most off the shelf active crossovers are housed inside a metal box, so you put it in a shelf, just like the rest of your hifi equipment.


But it's usually a tedious process deciding and calibrating the frequency cutoff for each driver. So have fun playing with your speakers...




And yea, the dream of owing a pair of those Nautilus speakers... One day, I tell ya all, one day, when I will be finally mad enough, and sell my house, and decide that living in a cardboard box shack is good enough for an audiophile, I'll finally be able to purchase a pair. Until that day...

 

[WmAx]

So, would it then follow that to maximise its effectiveness, the curvature of the cabinet around each driver would be different, given that each driver reproduces a separate range of frequencies (not saying that B&W have done this)?

Yes, this is true, from a minimum size perspective of that curvature. However, one curvature could be used, if it was of sufficiently large size.

So, does it then follow that in any speaker in which the drivers are decoupled from the cabinet, the drivers themselves must be made of a more rigid material than might typically be used in order to accomodate vibrations that ordinarily would be dispersed into the surrounding enclosure? If so, could this at least partially explain B&W's use of aluminium for the drivers over, say, paper?

The issue is not with the driver diaphragms, but with the frame assemblies. Ideally, the frame assembly should not resonate in the bandwidth that the speaker is used. Also, the suspension assembly(decoupling the speaker) can potentially have an interaction with the speaker, under specific circumstances, causing a small null in the response, but this is usually only when something is mechanically loose, preventing a stable coupling. When you de-couple the driver from the main enclosure mass, the vibrational energy is increased in the driver frame/motor by several magnitudes. You can reduce this energy by installing a heavy mass/weight that the driver is coupled to in the enclosure, as an anchor, but this anchor must be in turn de-coupled from the enclosure. But I would not worry so much about this detail, as the primary issue is to reduce exterior panel vibration, which can easily distort the timbre of the music, and this is one of the the most difficult things to address in speaker design, especially in terms of cabinet construction cost(s).

To what accuracy would you set the gain in order to consider all amps as outputting equally?

The standard maximum amplitude deviation allowed in perceptual tests is 0.1 dB. This deviation is considered to be impossible for a human to differentiate differences in loudness, even with the most sensitive test signal(white noise).

Hmm. So are you saying that you wouldn't connect all the power amps directly to the pre amp as in normal separates, but that all the components connect separately into the Nautilus' crossover and it then sorts everything out from there (I must admit to wondering how one would connect eight power amps into a pre amp from the point of view of never having seen so many pre-outs on a pre amp before)

Other posters already answered this question accurately.

Whilst looking through the gallery, I kept on seeing various speakers described as having full-range drivers yet typically there appeared to be only one driver which itself looked to be no more than 6" to 8" in diameter. How can the speakers be considered full-range?

Thanks mate.

The only thing that 'full range' means in that context, is that they are using a single driver for the entire bandwidth. I only pointed to that site in reference to Martin King, not in reference to the sub-optimal speaker systems featured in the gallery.

-Chris

 

[Buckle-meister]

...the primary issue is to reduce exterior panel vibration, which can easily distort the timbre of the music, and this is one of the the most difficult things to address in speaker design, especially in terms of cabinet construction cost(s).

Ok then, would it be correct to say that the two principal methods of reducing speaker enclosure vibrations are:

1. To decouple the drivers from the enclosure or;
2. Not to decouple the drivers, but to make the enclosure as massive and rigid as possible?

Correct?

 

[WmAx]

Ok then, would it be correct to say that the two principal methods of reducing speaker enclosure vibrations are:

1. To decouple the drivers from the enclosure or;
2. Not to decouple the drivers, but to make the enclosure as massive and rigid as possible?

Correct?

Ideally, you would use both in combination; this is what B&W practices on their top range offerings such as the Nautilus, N800, N801 and N802. The mid-range sections on these units are of very high rigidity, due to the geometry, and also highly damped, mechanically, due to the particular plastic composite that they use for the mid-range enclosure(s) on their top offerings. You can not depend on decoupling alone, as a substantial amount of energy is also transmitted via acoustical coupling from within the cabinet. Other techniques are to have a cabinet with a resonance frequency that is out of the pass-band, or a resonance frequency of very high Q(which is less audible as compared to typical broad Q resonances on standard cabinet panels).

-Chris